The present invention relates to a process for the purification of glycidyl methacrylate and acrylate. These compounds are known synthesis intermediates which are of particular interest because of their polymerizable acrylic double bond and their epoxy functional group.
Glycidyl methacrylate and acrylate (referred to hereinafter by the abbreviations GLYMA and GLYA) are generally obtained by the following two reaction stages,
(a) neutralization of methacrylic and acrylic acids respectively with a base such as an anhydrous alkali metal carbonate or hydrogen carbonate, with epichlorohydrin as solvent, which produces the solid acid salt which remains in suspension in epichlorohydrin, with release of carbon dioxide and water, according to the reaction scheme below (using an alkali metal carbonate): ##STR1## PA1 (b) the reaction of the acid salt with epichlorohydrin after introduction, as catalyst, of a quaternary ammonium salt such as trimethylbenzylammonium chloride, triethylbenzylammonium chloride, tetramethylammonium chloride and tetramethylammonium bromide, or a tertiary amine, such as triethylamine, tributylamine, triphenylamine, dimethylaniline or pyridine. The overall reaction scheme is as follows: ##STR2## To conduct this stage, the catalyst is introduced directly into the mixture obtained in the first stage, which comprises the acid salt formed, the solvent (which is a reactant from now on) and the excess starting carbonate, the temperature being of the order of 95.degree.-100.degree. C. The salt MCl precipitates and GLYMA or GLYA is formed, both being liquids. The final crude reaction mixture contains solid salts in suspension, namely the excess starting sodium carbonate and the MCl formed, which are separated off, for example by filtration. PA1 in a first stage, a distillation of the glycidyl (meth)acrylate to be purified is conducted in the presence of a first solvent capable of forming a low boiling point heteroazeotrope with the light impurities and epichlorohydrin, so as to obtain a head fraction which consists essentially of a solvent-light products heteroazeotrope; and PA1 in a second stage, the glycidyl (meth)acrylate thus freed from the light products is subjected to a distillation in the presence of a second solvent capable of forming a heteroazeotrope of low boiling point with glycidyl (meth)acrylate, so as to obtain a head fraction consisting essentially of the second solvent and the required pure glycidyl (meth)acrylate, thus freed from the heavy impurities,
(R.dbd.H, CH.sub.3) (N.dbd.alkali metal)
The reaction is conducted with an excess of carbonate relative to the starting acid in a reactor of the Grignard type, stirred and heated to a temperature of approximately 90.degree.-100.degree. C. By way of example, a reaction mixture is employed with an epichlorohydrin/carbonate/(meth)acrylic acid molar ratio of the order of 6/0.7/1 is employed. The water formed during the neutralization is removed as it is being formed, by heteroazeotropic distillation of the water-epichlorohydrin mixture, so as to avoid secondary reactions between the water and epichlorohydrin. The heteroazeotrope is condensed and is separated at room temperature, the upper aqueous phase is drawn off and the epichlorohydrin is recycled to the reactor, to obtain an acid salt remaining in suspension, in order to avoid obtaining pasty mixtures highly loaded with acid salts;
The filtered crude reaction mixture has, more particularly, the following composition:
______________________________________ Epichlorohydrin 70-80% by weight GLYMA or GLYA 15-25% by weight Heavy impurities (which have 2-5% by weight a boiling point &gt; that of GLY(M)A) Light impurities (which have 1-2% by weight a boiling point &lt; that of GLY(M)A) ______________________________________
To purify the GLYMA or the GLYA, in a conventional manner, the filtered crude reaction mixture is subjected to a batchwise fractional distillation to separate off the light impurities and epichlorohydrin (topping) and then the heavy impurities (tailing).
The problem which then arises is that of the polymerization of GLYMA or GLYA, which can take place as soon as the temperature reaches at least 70.degree. C. in the boiler. The polymerization presents the disadvantage of, on the one hand, fouling the whole apparatus, which results in a plant stoppage and, on the other hand, of resulting in a loss of product. This polymerization is related to the temperature, to the period for which this temperature has been applied, and to the addition or otherwise of at least one polymerization inhibitor. To solve this problem, the temperature can be lowered and/or the residence time in the boiler can be reduced. During the distillation the temperature rises and the pressure is reduced progressively to keep the temperature constant in the boiler. At the end of the epichlorohydrin distillation (topping) the absolute pressure is generally of the order of 1.33.times.10.sup.2 -2.66.times.10.sup.3 Pa (1-20 mm Hg) so as not to exceed 100.degree. C., as mentioned in DE-A-3,126,943 and FR-A-2,286,823.
Furthermore, as indicated above, at least one polymerization inhibitor is introduced into the boiler and at the head of the distillation column, such as hydroquinone, hydroquinone methyl ether or phenothiazine, which are effective in the presence of air, which is introduced continuously into the boiler.
Despite all these precautions, on the one hand, the problems of polymerization of GLYMA or GLYA in the boiler are not completely prevented and, on the other hand, the content of epichlorohydrin in the boiler after topping is still higher than or equal to 0.2%, that is to say too high.
To distil GLYMA or GLYA, by the usual technique, the absolute pressure in the boiler is of the order of 1.33.times.10.sup.2 -6.65.times.10.sup.2 Pa (1-5 mm Hg), as indicated in the examples of German Patent Application DE-A-3,126,943. Obtaining a high vacuum is an operation which is very constraining on an industrial scale and which requires the use of a perfectly leakproof plant. Similarly, the vacuum obtained in the boiler is also limited by the pressure drops of the distillation column.
Furthermore, during the tailing of GLYMA or GLYA, the exhaustion of the heavy residue in the boiler is also limited by the risks of polymerization as soon as the temperature in the boiler reaches at least approximately 70.degree. C.
To overcome these difficulties it is proposed, in accordance with Patent CA-A-986,126, to introduce an inert gas at the foot of the epichlorohydrin distillation column so that the remaining traces of epichlorohydrin are entrained by the inert gas. On the one hand, the introduction of such an inert gas is incompatible with the presence of a polymerization inhibitor and, on the other hand, gases contaminated by traces of epichlorohydrin, which are very difficult to purify, leave the plant. Epichlorohydrin is a toxic product whose residual contents in GLYMA or GLYA should preferably not exceed 100 ppm. Thus, the process according to Patent CA-A-986,126 would make it possible to solve the problem of residual epichlorohydrin--this being despite other difficulties, namely complicated or polluting plants--but, in no case would it solve the problem linked with the polymerization of GLYMA or GLYA.